In any lighting system, the illumination source and the optics used to direct and focus the light produced by the illumination source determine the lighting distribution produced by the lighting system. Likewise, to achieve a given lighting distribution there are only certain optics and illumination source combinations that will produce the desired lighting distribution. The design of the illumination source then governs the design of the optics that will produce the desired lighting distribution. The more specific the requirements are for the desired lighting distribution, the more difficulty is encountered in effectively controlling the lighting distribution.
Most of the current lighting technology uses a filament bulb as the source, with the optics consisting of a reflector, a lens, or a combination of a reflector and a lens. Reflectors that are designed for non-imaging optics use reflecting surfaces that are some form of second order polynomial or a conic surface. A parabolic reflector is the most common type and it will be used as an example herein, but the same principles apply to any conic reflector or piecewise conic reflector.
The shape of the illumination source determines the light distribution emitted from that illumination source. For example, a point source emits a spherical distribution. A filament emits a donut shape distribution for a simple straight filament. For a filament that curves or bends, the distribution is the sum of all the donut shaped distributions emitted in each straight section.
Since the package size, which is fixed by the allowable lamp geometry, is limited, the source location and design can improve the lamp efficiency. The value for the focal length of the reflector determines the depth and size of the reflector, which then determines the minimum size of an image in the distribution. The minimum illumination source image size is the smallest feature that is controllable in the lighting distribution. In some area lighting applications, this is not important. In applications with very specific lighting distributions, this can be critical.
This scaling of the reflector within a fixed size limit and lighting distribution directly affects the efficiency of the lamp. The efficiency of the lamp can be measured as the number of useable lumens (the amount of light) in the planned light distribution pattern divided by the number of lumens produced by the illumination source. The scaling of the focal length with a fixed size limit will produce one of three types of reflector.
Referring to FIG. 1, the depth and size of a reflector is determined by the focal length and the width limit for the lamp. A parabola with a long focal length will fill the available space without capturing all of the light, since the reflector does not extend as far as the latus rectum. The latus rectum 10 for a parabola 12 is a line through the focus 14 and perpendicular to the axis 16 of the parabola 12 (defined as parallel to directrix), as shown in FIG. 1. The width limit 18 of the lamp is dictated by the design criteria for the lamp. Since this reflector 12 would extend past the width limit 18 of the lamp before reaching the latus rectum 10, the width limit 18 prevents the use of a reflector 12 that would capture all of the light emitted by the hemispherical illumination source.
The next case has a decreased focal length so that the parabola 12 is deeper. The performance limit is the special case where the latus rectum 10 crosses the parabola 12 at the exact width limit 18 required for the lamp, as shown in FIG. 2. This reflector 12 would collect the maximum amount of light from the illumination source with no shaded areas. This gives the maximum collection of light for a hemispherical source but limits the light distribution that can be produced by the lamp since there is only one focal length for a given width. Unfortunately, the available space for a headlamp in practice rarely corresponds to this ideal situation. As illustrated in FIG. 3, the third case is to have a required focal length 14 that is less than that which is ideal for the width limit 18 of the lamp, resulting in all of the light from the illumination source being captured by the reflector 12, but much of the outer reaches of the reflector 12 receiving no light from the hemispherical illumination source.
For the case shown in FIG. 2, the maximum distance to the reflector 12 from the illumination source is at the latus rectum 10 and the shortest distance is at the vertex 20. The shortest distance to the reflector 12 determines the largest angle of projection of the source and the largest distance to the reflector 12 determines the smallest angle of projection of the source. For any given width in the case shown in FIG. 2, these distances and angles are fixed and can not be changed if they do not meet the requirements for a lamp. First, increasing the depth of the parabola 12 increases the light capture angle. Continuing to increase the depth of the parabola 12 eventually results in the sides of the reflector 12 extending beyond the latus rectum 10 of the parabola 12. Second, decreasing the distance between the illumination source and the reflector vertex 20 increases the angular size of the illumination source in the reflector 12, thus increasing the size of the image of the source in the design distribution. For a given focal length 14, the angular size of the illumination source in the reflector 12 decreases as the point of reflection moves from the vertex 20 to the latus rectum 10. This is because the cosine projected area of the illumination source is decreasing and the distance between the point of reflection and the illumination source is increasing, as shown in FIG. 4.
When the illumination source is a filament, continuing to move the illumination source past the latus rectum 10 initially increases the angular size of the source in the reflector 12 because the cosine projected area of the illumination source increases. However, eventually the increasing distance between the illumination source and the reflector 12 overcomes this and the angular size of the illumination source in the reflector 12 starts to decrease. (The cosine projected area of the illumination source reaches a minimum when the point of reflection is at the latus rectum 10 of the reflector 12.)
For general lighting, such as halogen bulb headlamps, the filament in the incandescent bulb can be oriented in any direction. In vehicular light applications, most bulbs have a straight filament that is oriented either Transverse or Axial. The Transverse Filament (TF) illumination source 22 has its wire or coil oriented perpendicular to the axis of symmetry 16 of the bulb (see FIG. 4). This in turn makes the filament 22 transverse or perpendicular to the optical axis. In most cases, since the bulb is usually viewed along the optical axis of the lamp, a TF source will project a different light distribution for reflection in the direction of the filament 22 than in the direction perpendicular to the axis of the filament 22.
As shown in FIG. 5, the filament 24 can also be mounted axially so that the focal length can be made as short as needed. However, even some of the emitted light 26 will never hit the parabolic reflector 12 and is therefore lost, as illustrated at 28 in FIG. 5. When using a hemispherical illumination source, such as an LED source, only half of the parabolic reflector 12 is in the hemisphere of emitted light 26. This is because LED light sources emit all of their light on one side of the source, into a hemisphere. The other side of an LED light source needs to be connected to a heat sink.
The image size produced from a section of a reflector 12 depends on the shape of the reflector 12 surface and the distance from the illumination source. The smallest image size is limited by the distance of the reflector 12 from the illumination source. Depending on the width limits 18 of the reflector 12, the maximum distance may be before the latus rectum 10, at it or beyond it. The focal length affects the shape and size of the reflecting surface. The longer the focal length, the larger the width of the reflector 12 becomes for a fixed distance from the vertex 20 or the latus rectum 10. The vertex 20 of the reflector 12 is the point where the optical axis 16 would cross the reflector 12. In the following figures, the maximum width will be fixed as it is in most practical cases. The lamp will stop at the point where the reflector parabola 12 crosses the vertical lines 18 (indicating the maximum allowed lamp width) in the figure.
Incandescent vehicle headlamps have been designed with two filaments in the bulb. Typically, one filament is used for the low beam and the other is used for the high beam. FIG. 6 illustrates several dual filament configurations for bulbs. Optical design techniques have been developed for a single reflector lamp that utilizes the offset of the filament sources to switch between low beam and high beam in the headlamp system. In the dual filament bulb, the filaments are offset from each other by a physical displacement. In most cases, one filament is designed to produce more light than the other. In the design of the optics, one filament is designed to be at the focus 14 and the other is offset from the focus 14 by some physical displacement distance. This offset in the illumination source position causes an offset in the light distributions produced by the two filaments. Such a lamp more effectively utilizes the reflector area for low and high beam operation.
Light emitting diode (LED) headlamps have used a combination of prior art techniques to project the image of LEDs into desired photometric distributions. Early designs used lens optics in conjunction with point sources. These proved to be difficult to design and manufacture, as the photometric distribution had to be constructed from point sources.
An advancement in LED packaging technology has since provided LEDs in an array. Examples of LED sources configured in arrays (linear and two-dimensional) are shown in FIGS. 7 and 8.
Prior art LED headlamps that use these arrays all use existing optical concepts currently used in incandescent or high intensity discharge (HID) lighting. These include lens optics, reflector optics, or a combination of the two. While no optical system can collect 100% of the emitted light, a good filament design can put 50% to 80% of the emitted light into the design distribution. As the currently disclosed technology focuses on reflector technology, the discussion of the prior art LED headlamps will address use of reflectors with LED arrays.
The current state of headlamp design is moving from incandescent bulbs to LED illumination sources. The orientation of the incandescent bulb's filament is either transverse or axial with respect to the optical axis of the illumination sources. The LED package (an array of dies or one single die) lends itself to a similar type of reflector design as the filament based lamps. The one major difference between a filament and an LED, however, is that a filament emits light in all directions when energized, while an LED emits light only in one hemisphere. Mounting an LED in one of the two standard axial or transverse orientations limits the collection efficiency or the size of the lamp, respectively. This has necessitated a different approach for LED forward lighting designs. One of those approaches has been to use lens-based optical systems, with the lens in front of the LED and directly imaging it onto the road. While this approach is mechanically simple, it is not very efficient at collecting all of the light from the LED. The most common reflector-based approach has been to use half of a reflector, with the LED mounted on the axis of the reflector and pointing to the side. This approach has the potential for high efficiency, but only by making the reflector very large or by sacrificing the ability to make a highly focused beam. Another reflector-based approach is to aim the LED directly back into the reflector. This has the advantage of being able to collect all of the light from the LED, but the disadvantage is that there is only one focal length for any given width and the LED mount blocks a significant portion of the light coming from the reflector. This disadvantage is made worse by the fact that it blocks the light coming from the center of the LED, which is where the highest light flux originates.
FIG. 9A illustrates the obvious disadvantage to mounting an LED array 24 in an axial position. Light in the area 28 misses the reflector 12. Light in the area 30 contacts the reflector 12 and contributes to the lighting distribution of the lamp. However, since light is only emitted on one side of the LED array 24 package, the reflector 12 on the other side provides no effect to the lighting distribution since it is in the region 32 where no light is emitted.
As shown in FIG. 9B, all of the light from a transverse mounted LED 22 can be collected as long as the reflective surface 12 fills the hemisphere 40 illuminated by the LED 22. The problem involves the relationship between the LED 22 location and the limits of the collecting optics. The sources 22 are usually placed at or near the focal point 14 of the optics. For an LED, the light stops at the latus rectum 10 since no light is emitted from the back side. Moving along the reflector 12 away from the optical axis, the image will shrink much faster for the LED than for the filament since the projected cross-section of the source is decreasing along with the increasing distance between the reflector surface and the illumination source, with the image eventually approaching zero. As the largest images will be produced from the vertex 20 of the reflector 12, this area is used for spread light. The small images near the latus rectum 10 are best used for the highly focused parts of the beam; however, in the transverse mounting condition, only a small amount of the luminous flux of the LED is emitted at such wide angles, limiting the brightness of the high intensity areas.
Therefore, there is a need for improved designs for headlamps. The present disclosure is directed toward meeting this need.